Bleaching of kraft pulps produced from green liquor pre-hydrolyzed South African Eucalyptus grandis wood chips (original) (raw)
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On the Recovery of Hemicellulose Before Kraft Pulping
Bioresources, 2012
To assess the feasibility of implementing hemicellulose recovery stages in kraft mills, Eucalyptus globulus wood samples were subjected to aqueous treatments with hot, compressed water (autohydrolysis processing) to achieve partial dissolution of xylan. Autohydrolyzed solids were subjected to kraft pulping under selected conditions to yield a pulp of low kappa number, and to an optimized TCF bleaching sequence made up of three stages (alkaline oxygen delignification, chelating, and pressurized hydrogen peroxide), with minimized additions of pulping and bleaching chemicals. The final product had a relatively low kappa number (1.4), 641 mL/g ISO intrinsic viscosity, and 86.4% brightness.
Bioresource Technology, 2011
Hemicelluloses in industrially produced pre-hydrolysis liquor (PHL) were precipitated with ethanol. These PHL-derived hemicelluloses (PHL-EH) and a commercial, pure birch wood xylan sample (powder form) (BWX) were bleached using chlorine dioxide (D 0 and D 1) and hydrogen peroxide (Ep) in the D 0 EpD 1 sequence, and the chemical compositions, molecular weights and charge densities of the treated samples were assessed. When applied to high-yield pulp (HYP) at 50 mg/g, 26 and 20 mg/g of the bleached PHL-EH and BWX, respectively, were adsorbed without significantly affecting paper properties. These results suggest that semi-bleached hemicelluloses could be used to increase the basis weight of paper products. Furthermore, an integrated process was proposed that converts the kraft-based dissolving pulp production process into a biorefinery unit with dissolving pulp, bleached hemicelluloses and lignin as main products.
Effect of Alkaline Hemicellulose Extraction on Kraft Pulp Fibers from Eucalyptus Grandis
The alkaline extraction of hemicelluloses from hardwoods prior to pulping, for further conversion to value-added products, seems to be a promising pathway for paper mills to increase profit and improve sustainability. However, the amount of hemi-cellulose extracted will be limited by the requirement to maintain pulp quality and pulp yield in comparison to existing pulping processes. The effects of NaOH concentration, temperature, and time on hemicellulose extraction of Eucalyptus grandis were studied using a statistical experimental design. Extracted wood chips were subjected to kraft pulping to evaluate the effect of the extraction on cooking chemicals, pulp quality, and handsheet paper strengths. The selective xylan recovery (12.4% dry mass) from E. grandis combined with low-cooking, active alkali charge, and less cooking time ad-vantaged the xylan extraction and subsequent modified kraft pulping process under the studied conditions. Pulp viscosity, breaking strength, and tensile index of handsheets were slightly improved.
The aim of this study was to further evaluate the viability of the integrated pulp mill biorefinery concept. On one side the properties of hot-water pre-extractions were studied, in order to determine the effect of extraction on soda pulping and bleachability. On the other side, the performance of two dry-strength agents on the paper properties obtained from extracted and un-extracted (control) samples were investigated. It was found that hemicelluloses removal of the depithed bagasse chips was 24.6% at 140 • C for 10 min with a solid to liquor ratio (S:L) of 1:8 (w/w), and that mass removal increased with extraction time and temperature. The hemicelluloses removal reached 65.5% at 170 • C for 30 min. The residual extracted bagasse were subjected to soda pulping at 160 • C for 1 h with 11, 14 and 17% active alkali charge and a S:L of 1:5 (w/w). Conventional elemental chlorine free bleaching (D 0 ED 1 ) sequence was also used to compare the results with the un-extracted ones. Compared with the control sample, the overall pulp yield for extracted bagasse increased considerably while Kappa number and rejects decreased moderately. In addition, yield and Kappa number of the pulps improved with increasing alkali charge from 11% to 17%. After pre-extraction, better brightness compared with the control pulp was also shown. However, hot-water extraction was found to negatively impact some pulp properties including decreases in burst and tensile indices while addition of chitosan and cationic starch could improve the strength properties. Overall, papers treated with chitosan gave superior mechanical properties compared to the papers treated with cationic starch.
Industrial & Engineering Chemistry Research, 2010
This study was to investigate the effect of modification of the kraft cooking process integrated with hemicellulose pre-extraction on the properties of pulp produced from softwood chips. Loblolly pine wood chips were extracted with pressurized hot water at an elevated temperature and then subjected to conventional and modified kraft pulping. A reductive pretreatment using sodium borohydride (SBH) in a mild alkaline sodium sulfide solution was adopted for a modification of kraft pulping preceded by water extraction. The presence of SBH in pretreatment caused a pronounced increase in pulp yield of the water pre-extracted kraft pulps. This reductive modification strategy for water-extracted kraft pulping was evaluated in two different ways including double extraction (SBH-alkaline extraction preceded by water extraction) followed by AQkraft pulping (DE-AQKP) and water extraction followed by pretreated AQ-kraft pulping (WE-SB-AQKP). The DE-AQKP using 1% SBH showed the same pulp yield as normal kraft cooking at 12% water-extraction weight loss along with approximately 3% of additional sugar extracts from the second extraction stream. In the case of WE-SB-AQKP with 1% SBH, the same yield as the kraft control was achieved around 14% water-extraction weight loss. The SBH demand for zero-yield-loss in pretreatment was in proportion to waterextraction weight loss in kraft pulping preceded by water extraction. Pulps from the modified processes showed faster PFI mill refining responses than the corresponding kraft pulps. No significant changes in paper strength properties were observed in handsheets prepared from the modified kraft pulping preceded by water extraction except for some reductions in tear strength.
Industrial Crops and Products, 2013
The aim of this study was to further evaluate the viability of the integrated pulp mill biorefinery concept. On one side the properties of hot-water pre-extractions were studied, in order to determine the effect of extraction on soda pulping and bleachability. On the other side, the performance of two dry-strength agents on the paper properties obtained from extracted and un-extracted (control) samples were investigated. It was found that hemicelluloses removal of the depithed bagasse chips was 24.6% at 140 • C for 10 min with a solid to liquor ratio (S:L) of 1:8 (w/w), and that mass removal increased with extraction time and temperature. The hemicelluloses removal reached 65.5% at 170 • C for 30 min. The residual extracted bagasse were subjected to soda pulping at 160 • C for 1 h with 11, 14 and 17% active alkali charge and a S:L of 1:5 (w/w). Conventional elemental chlorine free bleaching (D 0 ED 1 ) sequence was also used to compare the results with the un-extracted ones. Compared with the control sample, the overall pulp yield for extracted bagasse increased considerably while Kappa number and rejects decreased moderately. In addition, yield and Kappa number of the pulps improved with increasing alkali charge from 11% to 17%. After pre-extraction, better brightness compared with the control pulp was also shown. However, hot-water extraction was found to negatively impact some pulp properties including decreases in burst and tensile indices while addition of chitosan and cationic starch could improve the strength properties. Overall, papers treated with chitosan gave superior mechanical properties compared to the papers treated with cationic starch.
Cellulose Chemistry and Technology
Hemicelluloses were pre-extracted from giant bamboo with dilute H 2 SO 4 prior to Kraft and soda AQ pulping. The reaction conditions were selected to convert most of the hemicelluloses into soluble monomeric sugars, leaving almost unaltered the cellulose and lignin from the residual solid phase. A Central Composite Design was used to study four pre-extraction variables: H 2 SO 4 concentration (0.1-0.5% v/v), solid/acid solution ratio (1/3.5-1/5.5 g/mL), temperature (80-140 °C) and time (10-50 min). Temperature had a dominant influence on the hydrolysis process. A maximum xylose yield-of 83.4%-(based on oven dry raw material mass) was obtained at a 0.4% v/v H 2 SO 4 concentration, a solid/solution ratio of 1/4 (g/mL), at 140 °C and an extraction time of 40 min. The bamboo, from which 2.4% hemicellulose had been extracted, was subsequently pulped by both Kraft and soda AQ pulping methods. Soda AQ pulping gave the best results in terms of pulp yield, viscosity and kappa number, compared to the non-extracted bamboo, pulped under similar conditions.